Someone who uses a menstrual cup for a period instead of tampons or instead of/with disposable pads. One cup lasts 10+ years, its reusable and most are made with medical grade silicone or a similar material.

Conversion to a cup can be for financial reasons (average savings of ~$200-300/year or ~$2400-$3600 over 10 years),
or environmental (reusable), or for comfort/health (non absorbent/no chemicals)

Q: Can you spot me a tampon or pad?
A: Sorry, I don't use disposables, I'm a cupper.

The meaning is "loser". The word originates from the word "cup" which again gets its meaning from the grade of "U" which signifies a fail grade at IIT Madras. Therefore calling someone a cupper means that the person is a loser.

"What a cupper he is" or "Come on lets go to bruges, don't act like a cupper"